The present invention relates to the field of X-ray technology, namely to a CT-apparatus comprising an investigation bore with an X-ray source and an X-ray detector to generate an X-ray transmission image of an object to be investigated, a table with an elongated tabletop with a first longitudinal direction to support the object, which tabletop is translatable into the investigation bore in its first longitudinal direction, which tabletop comprises elongated mutually parallel insertions of a contrast material relative to the material of the tabletop for calibration purposes, said contrast material having a different X-ray attenuation coefficient than the material of the tabletop, said insertions having a second longitudinal direction.
A CT-apparatus of this type is known from U.S. Pat. No. 4,233,507. The CT-apparatus is generally used to perform an investigation of internal organs of a human body, based on the acquired X-ray transmission images from different directions. The human body comprises different tissues, which can be distinguished from each other due to the fact that such tissues have different radiation attenuation coefficients and, thus, have different characteristic X-ray absorption properties. It is important to precisely calibrate the CT-apparatus in order to be able to distinguish between tissues having similar radiation attenuation coefficients. In the known CT-apparatus an investigation table comprises calibration means to calibrate the Hounsfield""s numbers in order to perform a correct calculation of corresponding radiation attenuation coefficients. For that purpose a number of elongated insertions is provided in the tabletop, every insertion having a known X-ray attenuation coefficient, characteristic to a specific tissue. The diagnostic data acquired by a CT-apparatus is often used for further patient treatment, for example for radiotherapy. It is important, therefore to correctly assign the acquired X-ray transmission information to the topology of the patient. This problem is of specific importance when one uses a cone-beam CT acquisition mode. In this mode it is possible to produce X-ray transmission images in sequential (step and shoot) or in helical acquisition, while the tabletop is continuously translated during the X-ray exposure.
It is an object of the present invention to provide a CT apparatus where the accuracy of the tabletop position determination is improved. The CT-apparatus according to the invention is characterized in that the insertions are arranged with the second longitudinal direction orthogonally to the first longitudinal direction, wherein the spacing between the insertions in the first longitudinal direction is smaller than a dimension of the X-ray detector in the first longitudinal direction. The present invention is based on the insight that the tabletop position is defined within the system of the CT-apparatus, for example by a read-out of corresponding potentiometers with a certain tolerance, which is usually in the order of 2 mm. To improve this tolerance one can consider the read-out of the potentiometers as a coarse calibration of the tabletop position and the usage of insertions as a fine tuning of the tabletop position. Such insertions will have an a-priori known dimensions and spacing between them in the first longitudinal direction. In a cone-beam CT-apparatus one uses a two-dimensional X-ray detector, which comprises an array of detector elements. Thus, for this acquisition mode the maximum spatial resolution (i.e. the minimum spatial uncertainty) in the first longitudinal direction will be equal to the size of the detector element in this direction. After the data are acquired the back-projection is performed to reconstruct the transmission images. This image reconstruction uses the signals from the whole array of detector elements in the first longitudinal direction, due to the fact that the X-ray detector has a finite size in the first longitudinal direction and the acquisition is performed with a cone beam. After the reconstruction is performed a tabletop coordinate is prescribed to each resulting image. In order to improve the determination of the tabletop position the insertions are used. Therefore, the spacing between the insertions in the first longitudinal direction must be smaller than the detector size in this direction. The absolute values for the size and the spacing of the insertions are determined by the compromise between a substantial X-ray absorption within the insertions and a high spatial resolution. The size of the cross-section of the insertion is also determined by the material of the insertion. In general the spacing between the insertions will be in the range of 5 mm-20 mm. Due to the fact that the dimensions of the insertions as well as the spacing between them are a-priori known, one can overrule the potentiometer read-out of the tabletop position if an insertion is detected in a reconstructed image.
A further embodiment of the CT-apparatus according to the invention is characterized in that the insertions have a substantially rectangular cross-section. To calibrate the position of the tabletop one uses the known coordinate of, for example, the geometrical center of an insertion. However, it is also possible to use the edge to perform the coordinate calibration. In general, one can use insertions, which have a circular cross-section, however the advantage of using insertions with the rectangular cross-section is the convenience with which the edge detection algorithms can be fulfilled.
A further embodiment of the CT-apparatus according to the invention is characterized in that every insertion has a higher X-ray attenuation coefficient than the material of the tabletop. This embodiment ensures the optimal contrast between the insertions and the material of the tabletop, the latter being often fabricated from elements with low Z-value. Having insertions which have higher X-ray attenuation coefficient leaves the manufacturer a broader material choice. For example, Al is a material which can be used for manufacturing of these insertions.
A further embodiment of the CT-apparatus according to the invention is characterized in that detection means is provided to perform a detection of the position of any insertion on the X-ray transmission image. The insertion detection is performed on the lateral projection image, based on the insight that this projection provides the highest X-ray attenuation within the insertions and does not distort the geometry of the projection of the insertion. As an example of said detection one can mention well-known edge detection algorithms in case one calibrates the tabletop coordinate on the edge of the insertion, or edge detection algorithms in combination with Full Width of Half Maximum determination algorithms, if one calibrates the tabletop coordinate on the geometrical center of the insertion.
A further embodiment of the CT-apparatus according to the invention is characterized by first calibration means, which are provided to perform a calibration of the position of the tabletop in the first longitudinal direction. In this embodiment in order to acquire an optimum image of the marker the control means are available to perform a number of X-ray acquisitions per one degree of the X-ray source rotation. Further, the usage is made of, for example, a Least Square Fit algorithm to establish with a certain degree of confidence the spatial position of the insertion. Next, the calibration means overrule the read-out of the potentiometers in case an insertion is detected on the reconstructed image. If such correction is needed, the prescription means prescribe the coordinate of the insertion as the true coordinate of this reconstructed image.
A further embodiment of the CT-apparatus according to the invention is characterized by second calibration means, which are provided to perform a calibration of the position of the tabletop in the direction, substantially orthogonal to both the first and the second longitudinal directions. It is known from the clinical practice that in some cases the tabletop can bend substantially in some operational conditions. As has been explained earlier,
it is necessary to know the spatial position of the image plane with high accuracy. The second calibration means perform the calibration of the tabletop in a second direction, which often will coincide with the vertical direction. If a part of the tabletop is moved out of its original horizontal plane, the position of the insertion on the lateral transmission image will be lower than the expected one. By measuring the value of this displacement, one can correct for the tabletop movements out of its original plane.
The further embodiment of the CT-apparatus according to the invention is characterized by third calibration means, which are provided to perform a calibration of a tilt angle of the tabletop with respect to the plane of the transmission image. Due to the current medical practice it is required that the longitudinal axis of the tabletop is perpendicular to the plane of the X-ray source rotation. Small angle misalignments cause inaccuracies in the resulting image reconstruction. To detect and/or to correct for these small-angle misalignments the third calibration means perform the detection of the absolute dimensions of the cross-section of the insertions. If the tabletop is misaligned with the first longitudinal direction, the projection of insertions in the lateral direction will be distorted, resulting in a slightly enlarged image of an original geometric figure of a cross-section of the insertion with deteriorated edge sharpness. The third calibration means according to the invention can comprise a well-known algorithm for a Full Width of Half Maximum determination, in case one performs the misalignment detection by calculating the absolute dimensions of the cross-section of insertions. Alternatively, one can use an edge detection algorithm if one performs the misalignment detection by calculating the sharpness of the edge of the insertion.
These and other aspects of the invention will be discussed using the figures, where the corresponding numerals represent corresponding parts of the construction, wherein